The present invention relates to a thin film semiconductor gas sensor and manufacturing method of the same.
The gas sensor using metal oxide semiconductor, resistance-type gas sensor in which gas sensing material is coated on the ceramic substrate is widely used.
This resistance-type gas sensor is a thick-film sensor. FIG. 1 is a cross-sectional view of the gas sensor while FIGS. 2a & 2b are plan views of an electrode and a heater of the gas sensor.
As shown in FIG. 1, in the thick-film gas sensor, the electrode 1 to read an electric signal and gas sensing layer 2 which could react with detected gas are formed on the front surface of the a ceramic substrate 4. While, a heater 3 is formed on the back surface of the ceramic substrate 4 for heating gas sensing layer 2 to a predetermined temperature to enhance the reaction of the gas sensing layer 2 with detected gas. At this time, the ceramic substrate 4 is usually manufactured with alumina (Al.sub.2 O.sub.3) substrate of about 0.635 mm thick or alumina cylinder.
FIGS. 2a and 2b are plan view of the electrode 1 and the heater 3, in which the resistance-type gas sensor is formed on the back surface of the ceramic substrate 4 made of alumina and the heater 3 is formed by coating conductive material by screen printing method and calcining at high temperature. As for the conductive material for manufacturing the heater, RuO.sub.2 -based material, Ni--Cr-based material, W-Pt-based material, etc. were used.
The electrode 1 is formed by coating conductive material such as Pt on the front surface of the ceramic substrate 4 using screen printing method and then calcining at high temperature. The conductive material for the heater or electrode should have an appropriate viscosity for applying screen printing method and maintain viscous solution state in which various materials are mixed, in order to easily adhere to the substrate.
The gas sensing layer 2 having a thickness of from about several .mu.m to several tens .mu.m is formed by coating a mixture-type powder of gas sensing material and catalyst and a paste homogeneously mixed with organic solvent, on the ceramic substrate 4 and between the electrodes 2 by screen printing method and then calcining at 300.degree.-1,000.degree. C. to give the gas sensor.
The powder used as a gas sensing material of the gas sensing layer 2 is obtained by coprecipitation or is fine and homogeneous conventionally used powder. Generally, various material could be used for gas sensing layer 2. For example, SnO.sub.2 is usually used for detecting flammable gas, WO.sub.3 is for detecting alcohols and metal oxide semiconductor such as ZnO is for detecting various gas. Catalyst is added to the gas sensing layer 2 for enhancing the reaction with the detecting gas. Noble metals such as Au, Pt, Pd, etc. in various states could be added to the gas sensing layer 2 as the catalyst.
The paste for the gas sensing layer is prepared by pulverizing the gas sensing material and mixing thus obtained powder with organic solvent. The powder for the gas sensing material is prepared as follows.
First, adding an appropriate amount of catalyst to the gas sensing material and ball-milling the mixture to mix the gas sensing material and the catalyst homogeneously. That is, after mixing the gas sensing material with the catalyst in a constant ratio the mixture is ball-milled with alcohol and balls of appropriate size. The balls are separated from the powder through sieving the ball-milled mixture. The powder is sufficiently dried in an infrared drying apparatus to evaporate the alcohols. At this time, the catalyst should not be allowed to lump together to precipitate.
The dried powder is pulverized to an appropriate size using agate mortar, and then the powder is mixed with alcohols and balls to further pulverize the powder even finer. The pulverized powder is sieved to separate the balls and dried using an infrared drier to evaporate the alcohols. The dried powder is pulverized again in an agate mortar to obtain the final powder including catalyst.
The reason why the gas sensing material is pulverized to such a fine powder is to increase contacting surface area during the reaction with the detected gas and to increase the viscosity.
In the conventional gas sensor, oxides such as CuO are used as the catalyst, and when the adhesion state of the ceramic substrate and the gas sensing layer is not good, SiO.sub.2, Al.sub.2 O.sub.3 or etc. are added to the gas sensing material to compensate. SiO.sub.2, Al.sub.2 O.sub.3 or etc. also could be added to the gas sensing material in the same method as the catalyst addition to the gas sensing material.
Gas sensing material mixed with various materials is pasted with the organic solvent to obtain a gas sensing material paste having appropriate viscosity which could be easily applied to the screen printing method. As for the solvent, it is a solution prepared by dissolving 5-10% of ethylcellosolve in .alpha.-terpenol at 80.degree. C. with stirring.
FIG. 3 is a perspective view of the conventional sintered gas sensor.
In the ceramic cylinder-type gas sensor illustrated in FIG. 3, the gas sensing layer could not be formed by the screen printing method. Accordingly, gas sensing material, for example SnO.sub.2 paste is coated on the surface of the cylindrical ceramic tube 5 using brush to form gas sensing layer 6. Both electrodes 7-1 and 7-2 and the heater which were manufactured through the sequential process of forming the electrodes and the heater were used after connecting them with lead wire.
The thus-obtained gas sensor as illustrated in the FIGS. 1-3 are called resistance-type semiconductor gas sensor and operation mechanism thereof is as follows.
The principle of sensing the detected gas of the gas sensing layer is as follows.
Initially, various gases such as oxygen in the air are chemically bonded on the surface of the gas sensing layer. The chemically bonded gases make bondage through electrons in the semiconductor state gas sensing layer.
Therefore, the gas sensing layer is deficient in electrons and the conductivity thereof decreases. That is, the resistance of the gas layer remarkably increases owing to the increase of the resistance.
In this state, if detected gas exists, the adsorbed gases on the surface of the gas sensing layer through chemical bond reacts with the detecting gas to produce other gas and simultaneously the electrons bonded with the adsorbed gases on the surface of the sensing layer separate again. Accordingly, the conductivity of the gas sensing layer increases while the resistance of the gas layer remarkably decreases.
That is to say, in the resistance-type semiconductor gas detecting sensor, the resistance of the gas sensor decreases if the gas under detection exists, while the resistance increases if the gas under detection does not exists.
In case of constructing electric circuit by connecting this gas sensor with the load resistance in series and applying constant voltage, the current flowing in the series circuit is low when the gas under detection does not exist and the resistance of the gas sensor is high, while the current flowing in the series circuit gradually increases when the concentration of the gas under detection increases and the resistance of the gas sensor gradually decreases. Accordingly, the voltage at the load resistance increases. That is, the voltage at the load resistance varies according to the gas concentration.
The heater provided in the metal oxide semiconductor gas sensor emits heat as the current flows and heats the gas sensing layer to a predetermined temperature. The reason why the gas sensing layer is heated to the predetermined temperature is that the gas sensing layer has high selectivity to some gases according to the coated material. That is, through activating the gas sensing layer and heating it to constant temperature, reaction with the gas under detection is facilitated and side effects caused by humidity and other gases other than the detected gases could be reduced and the selectivity to the detected gas could be increased. In addition, the response time to the detected gas of the gas sensing layer could be shortened. And a contaminant could be removed from the surface of the gas sensing layer when the surface of the gas sensing layer is exposed to other gases and contaminated.
In the above described thick film-type gas sensor, since the ceramic substrate is provided between the heater for heating the gas sensing layer and a gas sensing layer, an effective heating of the gas sensing layer is difficult owing to large heat loss. Moreover, since the ceramic substrate is thick and it is difficult to form the heater accurately through the screen printing method, the temperature distribution in the gas sensing layer is non-uniform and error thereof is large. Therefore, the gas sensor often mis-operates and its reliability become worse.
Meanwhile, in case of forming both of the electrode and the heater on the front surface of the ceramic substrate, the manufacturing process is complicated let alone local heating owing to the uneven heater and the ceramic substrate. Accordingly, the locally occurred high temperature region accelerates the deterioration of the gas sensing layer and this induces a serious problem of shortening the lifetime of the gas sensing layer along with the deterioration of the response characteristic of the gas sensor. Moreover, in case of manufacturing the gas sensor using the ceramic substrate, it is difficult to form plural gas sensors with various gas sensing materials on the same substrate. In order to manufacture plural gas sensors on the same substrate, repeated screen printing should be carried out. If repeated screen printing is carried out, since the adhesion strength between the gas sensing layer and the ceramic substrate is weak, previously the screen printed gas sensing layer deteriorates. And screen printing a plurality of gas sensing layers in accurate position is very difficult. Moreover, the thus manufactured gas sensor is large-sized and so power consumption becomes high. Practically, when packaging the device, the connection of the lead wire of the electrode and the heater to the fixing pin in the package is difficult.
Therefore, the conventional gas sensors usually consist of single gas sensor, and in this case, the problem of showing non-selectivity that the gas sensor reacts with various gases does not occur.
To solve the non-selectivity problem on the detected gas, a technique of forming a plurality of gas sensor on a silicon substrate using semiconductor technique is developed.
However, though the various problems could be solved when manufacturing the gas sensor of plurality of thin film by applying the semiconductor manufacturing technique, since each gas sensor has different operation temperature which varies according to the material of the gas sensing layer of the gas sensor, manufacturing method thereof and kind of the gas under detection, a separate heater should be provided. And since different electric power should be applied to each heater, the same number of sources of electric power to drive each heater are needed. That is, in order to drive a small array gas sensor, a plurality of seperate large-sized sources of electric power are needed.
Generally, the gas sensor when compared with other devices is liable to be affected by the ambient factor, gases as well as by other factors other than the gases such as humidity or temperature. Moreover, the initial resistance of the air of the gas sensing layer gradually varies through heating by the heater.
The initial resistance indicated by the sensor in the fresh air is the reference of all the information data when using the pattern recognition method which consists plurality of gas sensor in a chip on the same substrate to distinguish gases and to react with specific gas selectively. Therefore, when the change of the initial resistance according to the operation time is compensated, mis-operation of the sensor could be prevented owing to the error.
However, the conventional gas sensor does not have good selectivity and has serious problem of decreasing reliability owing to the mis-operation resulted from the change of the initial resistance through the external condition change with time or through heating by the internally provided heater.